[suicideneil]

One would gear down when using higher voltage, otherwise you will be spinning the wheels to fast to control the truck.

I think your genral issue is that the motor you have is too fast for 4s & racing (4s & bashing would be fun though), yet on 3s it will be pulling quite a few amps if not geared conservatively; most people tend to run the 1515 1y on 4s, or the next slowest motor option on 4s/5s.

I would stick with 3s if its working good for you, just gear as required to get your desired speed and 'oomph' levels vs temps.